feat: add emoji reaction support for files

- Implement `ReactionService` to manage file reactions in the database.
- Add `POST /d/{boxID}/f/{fileID}/react` endpoint to handle user reactions.
- Add `GET /emoji/{pack}/{file}` endpoint to serve custom emoji assets.
- Support loading custom emoji packs dynamically from the data directory.
- Update README with instructions on configuring emoji reaction packs.

backend/static/js/12-reactions.js

@@ -0,0 +1,198 @@
+(function () {
+  const picker = document.querySelector("[data-reaction-picker]");
+  const panel = picker ? picker.querySelector(".reaction-picker-panel") : null;
+  const search = picker ? picker.querySelector("[data-reaction-search]") : null;
+  const closeButton = picker ? picker.querySelector("[data-reaction-close]") : null;
+  const tabs = picker ? Array.from(picker.querySelectorAll("[data-reaction-tab]")) : [];
+  const panels = picker ? Array.from(picker.querySelectorAll("[data-reaction-panel]")) : [];
+
+  let activeButton = null;
+  let activeCard = null;
+
+  document.querySelectorAll("[data-reaction-button]").forEach((button) => {
+    button.addEventListener("click", (event) => {
+      event.preventDefault();
+      event.stopPropagation();
+      openPicker(button);
+    });
+  });
+
+  if (!picker || !panel) {
+    return;
+  }
+
+  // Aurora's glass card uses backdrop-filter, and the main content animates
+  // with transform. Both can create a containing block for fixed descendants,
+  // so keep the floating picker at body level where viewport coordinates mean
+  // what they say.
+  document.body.appendChild(picker);
+
+  picker.addEventListener("click", (event) => {
+    if (event.target === picker) {
+      closePicker();
+    }
+  });
+
+  panel.addEventListener("click", async (event) => {
+    const emoji = event.target.closest("[data-emoji-id]");
+    if (!emoji || !activeButton || !activeCard) {
+      return;
+    }
+    await submitReaction(emoji);
+  });
+
+  tabs.forEach((tab) => {
+    tab.addEventListener("click", () => {
+      setActiveTab(tab.dataset.reactionTab);
+    });
+  });
+
+  if (search) {
+    search.addEventListener("input", () => filterEmoji(search.value));
+  }
+
+  if (closeButton) {
+    closeButton.addEventListener("click", closePicker);
+  }
+
+  document.addEventListener("click", (event) => {
+    if (picker.hidden) {
+      return;
+    }
+    if (panel.contains(event.target) || event.target.closest("[data-reaction-button]")) {
+      return;
+    }
+    closePicker();
+  });
+
+  document.addEventListener("keydown", (event) => {
+    if (event.key === "Escape") {
+      closePicker();
+    }
+  });
+
+  window.addEventListener("resize", () => {
+    if (activeButton && !picker.hidden) {
+      positionPicker(activeButton);
+    }
+  });
+
+  function openPicker(button) {
+    activeButton = button;
+    activeCard = button.closest("[data-reaction-card]");
+    picker.hidden = false;
+    picker.classList.add("is-open");
+    if (search) {
+      search.value = "";
+      filterEmoji("");
+    }
+    positionPicker(button);
+  }
+
+  function closePicker() {
+    picker.hidden = true;
+    picker.classList.remove("is-open", "is-mobile");
+    document.documentElement.classList.remove("reaction-picker-open");
+    picker.style.left = "";
+    picker.style.top = "";
+    activeButton = null;
+    activeCard = null;
+  }
+
+  function positionPicker(button) {
+    if (isMobilePicker()) {
+      picker.classList.add("is-mobile");
+      document.documentElement.classList.add("reaction-picker-open");
+      picker.style.left = "0px";
+      picker.style.top = "0px";
+      return;
+    }
+
+    picker.classList.remove("is-mobile");
+    document.documentElement.classList.remove("reaction-picker-open");
+    picker.style.left = "0px";
+    picker.style.top = "0px";
+    const buttonRect = button.getBoundingClientRect();
+    const pickerRect = panel.getBoundingClientRect();
+    const margin = 10;
+    const preferredLeft = buttonRect.left + (buttonRect.width / 2) - (pickerRect.width / 2);
+    const preferredTop = buttonRect.bottom + 8;
+    const left = Math.min(Math.max(margin, preferredLeft), window.innerWidth - pickerRect.width - margin);
+    const top = Math.min(Math.max(margin, preferredTop), window.innerHeight - pickerRect.height - margin);
+    picker.style.left = `${left}px`;
+    picker.style.top = `${top}px`;
+  }
+
+  function isMobilePicker() {
+    return window.matchMedia("(max-width: 820px), (pointer: coarse)").matches;
+  }
+
+  function setActiveTab(tabID) {
+    tabs.forEach((tab) => {
+      const active = tab.dataset.reactionTab === tabID;
+      tab.classList.toggle("is-active", active);
+      tab.setAttribute("aria-selected", active ? "true" : "false");
+    });
+    panels.forEach((item) => {
+      item.classList.toggle("is-active", item.dataset.reactionPanel === tabID);
+    });
+  }
+
+  function filterEmoji(value) {
+    const query = value.trim().toLowerCase();
+    picker.querySelectorAll("[data-emoji-id]").forEach((button) => {
+      const haystack = `${button.dataset.emojiId} ${button.dataset.emojiLabel}`.toLowerCase();
+      button.hidden = query !== "" && !haystack.includes(query);
+    });
+  }
+
+  async function submitReaction(emoji) {
+    const body = new URLSearchParams();
+    body.set("emoji_id", emoji.dataset.emojiId);
+
+    activeButton.disabled = true;
+    const response = await fetch(activeButton.dataset.reactUrl, {
+      method: "POST",
+      headers: {
+        "Accept": "application/json",
+        "Content-Type": "application/x-www-form-urlencoded",
+      },
+      body,
+    });
+
+    if (!response.ok) {
+      activeButton.disabled = false;
+      closePicker();
+      return;
+    }
+
+    const payload = await response.json();
+    renderReactions(activeCard, payload.reactions || []);
+    activeButton.remove();
+    closePicker();
+  }
+
+  function renderReactions(card, reactions) {
+    const list = card.querySelector("[data-reaction-list]");
+    if (!list) {
+      return;
+    }
+    list.replaceChildren();
+    reactions.forEach((reaction) => {
+      const pill = document.createElement("span");
+      pill.className = "reaction-pill";
+      pill.title = reaction.label || reaction.emojiId;
+
+      const image = document.createElement("img");
+      image.src = reaction.url;
+      image.alt = reaction.label || reaction.emojiId;
+      image.loading = "lazy";
+
+      const count = document.createElement("span");
+      count.textContent = reaction.count;
+
+      pill.append(image, count);
+      list.append(pill);
+    });
+  }
+})();