refactor(css): remove duplicated rules and fix three destructive-butt…

[olavgg]

…on hovers

app.css had accumulated rules written twice: sibling components with byte-identical declaration blocks, and one full second copy of every theme palette. Deduped without changing what renders — verified by diffing the flattened rule set (every selector expanded out of its selector list, declarations compared as a set) against HEAD, and by hand-checking cascade order wherever equal-specificity rules compete.

Merged, one skin instead of N copies:

• Archive / Leave / Delete-forever: 6 blocks that existed three times
• Sidebar rows: .channel-list and .dm-list shared hover, active, unread, muted and name truncation — six blocks that existed twice
• Search dropdown and /search results: the "not joined" pill, the
  tint, file-chip icons, snippet block-spacing
• .profile-content/.admin-content, the poll/about modal backdrops, .logo-mark, .send-icon sizing, the ADMIN role pill

Theme swatches now read the theme's own tokens. The swatch element carries that theme's data-theme and the [data-theme] blocks are attribute-only selectors, so it already resolves the right values; the 60 hardcoded hex codes were a second copy of every palette — two places to change a colour and one place to forget. A theme added to the picker now gets a correct swatch for free. All 60 stripes verified to resolve to the colour they did before.

Removed as dead: a second padding on .day-divider span, a duplicate .message { position: relative }, .composer-preview's restatement of .message-body, min-width: 0 on .content inside a media query where the base rule already set it, repeated cursor: pointer on emoji-picker hovers, and a comment paragraph printed twice.

Three var() references to custom properties that have never existed (--hover-bg, --surface, --text-muted) always rendered their fallback. Collapsed to that value so the code says what it does, with a comment naming the real token; the colours are unchanged.

Fixes: archive, admin-danger and call-end buttons lost their colour on hover. Each declared only filter: brightness(), so the background fell through to button:hover (0,1,1) or .call-btn:hover (0,2,0), which out-specify a single class — the two red buttons pressed in a hurry (decline, hang up) and Suspend/Clear messages repainted accent blue or a neutral tint. .channel-leave-danger and .channel-destroy-danger already restated their own background inside :hover; the same fix was applied twice and missed three times, which is the drift the duplication hid.