style(web): fix prettier config, format entire web/ tree

.prettierrc.json was missing "semi": false, so prettier wanted to add
semicolons to a codebase written without them (763 semicolon-free
statements vs. 150 with, in hand-written .ts; zero hand-written .svelte
files use them at all). That's why prettier --check failed on 249 files
— not because the code was unformatted, but because the config didn't
match the actual house style. Added "semi": false; left printWidth/etc
as configured (printWidth barely moves the failure count: 218/213/212
files at 100/120/140).

Ran `prettier --write .` with the corrected config. Verified
semantics-preserving before and after:
- eslint: 142 problems both before and after, byte-identical
- build passes, 38/38 tests pass
- token-stream diff (whitespace/semicolons/quotes normalized) on all
  218 changed files: only 52 had any remaining token change, all either
  trailing-comma removal (matching trailingComma: "none") or import/
  ternary reflow — no semantic changes
- live smoke test: Knowledge, Tasks, Fleet map, and a chat window
  (AgentTrace, markdown, Scope graph, activity rail) all render
  correctly, no console errors

Most of the diff is shadcn/ui vendor files (lib/components/ui/) moving
from the CLI's own style (double quotes, tabs, semicolons) to house
style; re-running `shadcn-svelte add` on a component will need a
follow-up format pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/src/lib/stores/background.svelte.ts

@@ -57,7 +57,12 @@ function storedConfig(): BackgroundConfig {
     return {
       pattern: VALID_IDS.has(parsed.pattern) ? parsed.pattern : DEFAULTS.pattern,
       color: isHexColor(parsed.color) ? parsed.color : DEFAULTS.color,
-      fillColor: parsed.fillColor === null ? null : isHexColor(parsed.fillColor) ? parsed.fillColor : DEFAULTS.fillColor,
+      fillColor:
+        parsed.fillColor === null
+          ? null
+          : isHexColor(parsed.fillColor)
+            ? parsed.fillColor
+            : DEFAULTS.fillColor,
       opacity: clamp01(parsed.opacity, DEFAULTS.opacity),
       fade: clamp01(parsed.fade, DEFAULTS.fade),
       scale: clampScale(parsed.scale),