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/views/GraphView.vue

@@ -600,7 +600,7 @@ onUnmounted(() => {
 .graph-page {
   display: flex;
   flex-direction: column;
-  height: calc(100vh - var(--header-height, 52px));
+  height: calc(100vh - var(--header-height));
   overflow: hidden;
 }
 
@@ -750,7 +750,7 @@ onUnmounted(() => {
   background: var(--color-bg-card);
   border: 1px solid var(--color-border);
   border-radius: var(--radius-md);
-  box-shadow: 0 4px 16px var(--color-shadow, rgba(0, 0, 0, 0.15));
+  box-shadow: 0 4px 16px var(--color-shadow);
   padding: 0.5rem 0.75rem;
   pointer-events: none;
   z-index: 10;