refactor(theme): remove 184 var() fallbacks — every one was unreachable

#2277 counted ~150 "raw colour literals bypassing the tokens". Measuring them
told a different story: 184 sat in `var(--token, #fallback)` position, and a
check against theme.css shows every one of those tokens IS declared. So the
fallbacks could not render. Not drift — vestigial.

They were also not this palette. The most common were Tailwind and Flat-UI
defaults — #6366f1 indigo, #22c55e green, #f59e0b amber, #3b82f6 blue,
#e74c3c and #27ae60 — a second, unsanctioned colour scheme sitting in the
codebase looking like the app's colours to anyone reading it.

Removing them is not tidying. #2319's lesson is that a fallback is WORSE than a
missing token: a missing token renders as nothing and someone eventually
notices, while a fallback renders something plausible forever. These 184 were
one token rename away from silently repainting the app in Tailwind. The design
token check would catch the rename — but the fallback is precisely the thing
that would make it invisible if the check were ever bypassed.

Literal count 152 -> 45, which matters beyond the number: a report that is
mostly unreachable noise is one people stop reading, and then it stops working
while still passing. What remains should be genuinely worth looking at.

Done with a paren-aware transform, not a regex — `var(--x, rgba(0,0,0,.5))`
nests parens and `[^)]+` would cut at the first one and leave `))` behind.
Verified after: every changed line is a fallback strip and nothing else, and
every var() reference still resolves.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

frontend/src/components/InlineAssistPanel.vue

@@ -135,8 +135,8 @@ const markers: Record<DiffLine["type"], string> = {
   flex-shrink: 0;
 }
 .iap-btn-cancel:hover {
-  border-color: var(--color-danger, #e74c3c);
-  color: var(--color-danger, #e74c3c);
+  border-color: var(--color-danger);
+  color: var(--color-danger);
 }
 
 .iap-stream-preview {
@@ -191,19 +191,19 @@ const markers: Record<DiffLine["type"], string> = {
   font-weight: var(--fs-weight-medium);
 }
 .iap-btn-accept {
-  background: var(--color-success, #22c55e);
+  background: var(--color-success);
   color: var(--fs-text-on-action);
 }
 .iap-btn-accept:hover { opacity: 0.85; }
 
 .iap-btn-reject {
-  background: var(--color-bg-card, var(--color-bg));
+  background: var(--color-bg-card);
   color: var(--color-text-secondary);
   border: 1px solid var(--color-border);
 }
 .iap-btn-reject:hover {
-  border-color: var(--color-danger, #e74c3c);
-  color: var(--color-danger, #e74c3c);
+  border-color: var(--color-danger);
+  color: var(--color-danger);
 }
 
 /* ── Diff ── */
@@ -226,12 +226,12 @@ const markers: Record<DiffLine["type"], string> = {
 
 .iap-diff-equal  { color: var(--color-text-muted); }
 .iap-diff-delete {
-  background: color-mix(in srgb, var(--color-danger, #e74c3c) 10%, transparent);
-  color: var(--color-danger, #e74c3c);
+  background: color-mix(in srgb, var(--color-danger) 10%, transparent);
+  color: var(--color-danger);
 }
 .iap-diff-insert {
-  background: color-mix(in srgb, var(--color-success, #22c55e) 10%, transparent);
-  color: var(--color-success, #22c55e);
+  background: color-mix(in srgb, var(--color-success) 10%, transparent);
+  color: var(--color-success);
 }
 
 .iap-diff-marker {