ci: tighten tracker-tag lint to exclude digit-runs glued to a letter (#1555)

The tracker-tag lint's trailing class `[^0-9a-fA-F]` treated a non-hex
letter as a valid terminator, so it flagged hash-route fragments like
#24h / #1h (the stats-page window-selector routes) as tracker tags — four
false positives in the frontend. A real tracker tag is never immediately
followed by a letter, so widen the trailing class to [^0-9a-zA-Z]: still
catches every real tag (followed by space / punctuation / EOL) while
excluding any digit-run glued to a letter. Letter-bearing and 6-8-digit
hex stay excluded as before.

Also write theme.css's --crust fallback in six-digit form (#000 -> #000000)
to dodge the one remaining pure-numeric-short-hex residual the lint
documents. Pixel-identical colour. With both, the frontend tree
contributes zero lint hits; only the legacy backlog (Rust) remains.

Pattern + cases reviewed and approved by atlas on the issue.

frontend/packages/shared/src/theme.css

@@ -29,7 +29,7 @@
      a swap. Each is pixel-identical to its prior literal under the
      default palette: --crust is a darkened bg; --muted / --subtext0 are
      foreground↔background blends (two levels of dimmed text). */
-  --crust: color-mix(in srgb, var(--base00) 58%, #000); /* terminal / code bg, below --bg */
+  --crust: color-mix(in srgb, var(--base00) 58%, #000000); /* terminal / code bg, below --bg */
   --muted: color-mix(in srgb, var(--base05) 55.5%, var(--base00)); /* secondary / dimmed text */
   --subtext0: color-mix(in srgb, var(--base05) 77.7%, var(--base00)); /* toolbar/status text; dimmer than --fg, lighter than --muted */
 }