feat(preview): add file preview page with metadata and styling

Implement a rich file preview interface to allow users to view file
contents directly in the browser.

Changes include:
- Exposing raw file size (`SizeBytes`) in the download handler's file view.
- Adding comprehensive CSS styling for the preview layout and cards.
- Integrating Prism.js for syntax highlighting of code files.
- Updating Content Security Policy (CSP) headers to permit inline styles and frame sources required by the preview components.
- Adding unit tests to ensure preview metadata attributes are correctly rendered on the download page.

backend/static/lib/prismjs/prism.css

@@ -0,0 +1,4 @@
+/* PrismJS 1.30.0
+https://prismjs.com/download#themes=prism-dark&languages=markup+css+clike+javascript+abap+abnf+actionscript+ada+agda+al+antlr4+apacheconf+apex+apl+applescript+aql+arduino+arff+armasm+arturo+asciidoc+aspnet+asm6502+asmatmel+autohotkey+autoit+avisynth+avro-idl+awk+bash+basic+batch+bbcode+bbj+bicep+birb+bison+bnf+bqn+brainfuck+brightscript+bro+bsl+c+csharp+cpp+cfscript+chaiscript+cil+cilkc+cilkcpp+clojure+cmake+cobol+coffeescript+concurnas+csp+cooklang+coq+crystal+css-extras+csv+cue+cypher+d+dart+dataweave+dax+dhall+diff+django+dns-zone-file+docker+dot+ebnf+editorconfig+eiffel+ejs+elixir+elm+etlua+erb+erlang+excel-formula+fsharp+factor+false+firestore-security-rules+flow+fortran+ftl+gml+gap+gcode+gdscript+gedcom+gettext+gherkin+git+glsl+gn+linker-script+go+go-module+gradle+graphql+groovy+haml+handlebars+haskell+haxe+hcl+hlsl+hoon+http+hpkp+hsts+ichigojam+icon+icu-message-format+idris+ignore+inform7+ini+io+j+java+javadoc+javadoclike+javastacktrace+jexl+jolie+jq+jsdoc+js-extras+json+json5+jsonp+jsstacktrace+js-templates+julia+keepalived+keyman+kotlin+kumir+kusto+latex+latte+less+lilypond+liquid+lisp+livescript+llvm+log+lolcode+lua+magma+makefile+markdown+markup-templating+mata+matlab+maxscript+mel+mermaid+metafont+mizar+mongodb+monkey+moonscript+n1ql+n4js+nand2tetris-hdl+naniscript+nasm+neon+nevod+nginx+nim+nix+nsis+objectivec+ocaml+odin+opencl+openqasm+oz+parigp+parser+pascal+pascaligo+psl+pcaxis+peoplecode+perl+php+phpdoc+php-extras+plant-uml+plsql+powerquery+powershell+processing+prolog+promql+properties+protobuf+pug+puppet+pure+purebasic+purescript+python+qsharp+q+qml+qore+r+racket+cshtml+jsx+tsx+reason+regex+rego+renpy+rescript+rest+rip+roboconf+robotframework+ruby+rust+sas+sass+scss+scala+scheme+shell-session+smali+smalltalk+smarty+sml+solidity+solution-file+soy+sparql+splunk-spl+sqf+sql+squirrel+stan+stata+iecst+stylus+supercollider+swift+systemd+t4-templating+t4-cs+t4-vb+tap+tcl+tt2+textile+toml+tremor+turtle+twig+typescript+typoscript+unrealscript+uorazor+uri+v+vala+vbnet+velocity+verilog+vhdl+vim+visual-basic+warpscript+wasm+web-idl+wgsl+wiki+wolfram+wren+xeora+xml-doc+xojo+xquery+yaml+yang+zig&plugins=line-numbers */
+code[class*=language-],pre[class*=language-]{color:#fff;background:0 0;text-shadow:0 -.1em .2em #000;font-family:Consolas,Monaco,'Andale Mono','Ubuntu Mono',monospace;font-size:1em;text-align:left;white-space:pre;word-spacing:normal;word-break:normal;word-wrap:normal;line-height:1.5;-moz-tab-size:4;-o-tab-size:4;tab-size:4;-webkit-hyphens:none;-moz-hyphens:none;-ms-hyphens:none;hyphens:none}@media print{code[class*=language-],pre[class*=language-]{text-shadow:none}}:not(pre)>code[class*=language-],pre[class*=language-]{background:#4c3f33}pre[class*=language-]{padding:1em;margin:.5em 0;overflow:auto;border:.3em solid #7a6651;border-radius:.5em;box-shadow:1px 1px .5em #000 inset}:not(pre)>code[class*=language-]{padding:.15em .2em .05em;border-radius:.3em;border:.13em solid #7a6651;box-shadow:1px 1px .3em -.1em #000 inset;white-space:normal}.token.cdata,.token.comment,.token.doctype,.token.prolog{color:#997f66}.token.punctuation{opacity:.7}.token.namespace{opacity:.7}.token.boolean,.token.constant,.token.number,.token.property,.token.symbol,.token.tag{color:#d1939e}.token.attr-name,.token.builtin,.token.char,.token.inserted,.token.selector,.token.string{color:#bce051}.language-css .token.string,.style .token.string,.token.entity,.token.operator,.token.url,.token.variable{color:#f4b73d}.token.atrule,.token.attr-value,.token.keyword{color:#d1939e}.token.important,.token.regex{color:#e90}.token.bold,.token.important{font-weight:700}.token.italic{font-style:italic}.token.entity{cursor:help}.token.deleted{color:red}
+pre[class*=language-].line-numbers{position:relative;padding-left:3.8em;counter-reset:linenumber}pre[class*=language-].line-numbers>code{position:relative;white-space:inherit}.line-numbers .line-numbers-rows{position:absolute;pointer-events:none;top:0;font-size:100%;left:-3.8em;width:3em;letter-spacing:-1px;border-right:1px solid #999;-webkit-user-select:none;-moz-user-select:none;-ms-user-select:none;user-select:none}.line-numbers-rows>span{display:block;counter-increment:linenumber}.line-numbers-rows>span:before{content:counter(linenumber);color:#999;display:block;padding-right:.8em;text-align:right}