[feat](trx-frontend-http): implement frontend styling & performance improvements

CSS: reduce backdrop-filter to modals only, add contain/content-visibility
for inactive tabs, optimize transitions to background-color, pre-compute
color-mix results, add container queries, split themes to lazy-loaded file.

JS: cache DOM refs in render path, add field-level diffing for SSE updates,
replace innerHTML with replaceChildren() in hot paths, add WebGL colour
cache invalidation on theme switch.

HTML: add defer to scripts, lazy-load plugin scripts on tab activation,
SVG sprite sheet for tab icons, template elements for deferred tab content,
improve aria-live/keyboard nav/colour contrast accessibility.

Server: upgrade Cache-Control to immutable, add Brotli compression alongside
gzip with Accept-Encoding negotiation.

Implements all items from docs/frontend_improvements.md except app.js ES
module split (P1, requires major refactor) and Web Worker migration (P3).

https://claude.ai/code/session_015rQNMGvusj5jY66MPUgYqt
Signed-off-by: Claude <noreply@anthropic.com>

Cargo.lock

@@ -31,7 +31,7 @@ dependencies = [
  "actix-utils",
  "base64",
  "bitflags 2.10.0",
- "brotli",
+ "brotli 8.0.2",
  "bytes",
  "bytestring",
  "derive_more 2.0.1",
@@ -414,6 +414,17 @@ dependencies = [
  "generic-array",
 ]
 
+[[package]]
+name = "brotli"
+version = "7.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cc97b8f16f944bba54f0433f07e30be199b6dc2bd25937444bbad560bcea29bd"
+dependencies = [
+ "alloc-no-stdlib",
+ "alloc-stdlib",
+ "brotli-decompressor 4.0.3",
+]
+
 [[package]]
 name = "brotli"
 version = "8.0.2"
@@ -422,7 +433,17 @@ checksum = "4bd8b9603c7aa97359dbd97ecf258968c95f3adddd6db2f7e7a5bef101c84560"
 dependencies = [
  "alloc-no-stdlib",
  "alloc-stdlib",
- "brotli-decompressor",
+ "brotli-decompressor 5.0.0",
+]
+
+[[package]]
+name = "brotli-decompressor"
+version = "4.0.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a334ef7c9e23abf0ce748e8cd309037da93e606ad52eb372e4ce327a0dcfbdfd"
+dependencies = [
+ "alloc-no-stdlib",
+ "alloc-stdlib",
 ]
 
 [[package]]
@@ -3117,6 +3138,7 @@ version = "0.1.0"
 dependencies = [
  "actix-web",
  "actix-ws",
+ "brotli 7.0.0",
  "bytes",
  "dirs",
  "flate2",