Scale stage 1: everything the UI sizes with is now rem

Groundwork for the continuous interface scale (a root font-size lever): every
size the UI derives from must be root-relative, or it freezes while the rest
grows. Gate for this commit: at the default 16px root the app renders
pixel-identical to before — every conversion is exact (11px = 0.6875rem …).

- The 8 panel-chrome tokens in styles.css: px -> rem, and the data-text
  compact/large tiers are DELETED — a continuous scale replaces discrete
  tiers. Same for --w7-fs (12px -> 0.75rem) and the win7 --radius (5px ->
  0.3125rem, still landing rounded-md exactly on --w7-el-bdr's 3px).
  --radius-sm/md subtrahends: px -> rem, or corners would sharpen as the UI
  grew.
- All 93 text-[11px]/text-[10px] arbitraries -> rem equivalents, plus the two
  inline padding:24px empty-state cells in the 7like file panel.
- 7like.css gains a scale bridge in skinfix: 7.css is 370 px values with zero
  rem, and --w7-font reaches only 15 rules' TYPOGRAPHY — under a root lever
  win7 text would grow inside boxes frozen at 23px/14px/15px. The bridge
  re-expresses 7.css's size vars and its hardcoded box metrics (buttons via
  .w7-btn only, inputs, table rows/headers, title-bar controls, menu items,
  tabs, the 10pt check glyph) in rem at identical scale-1 pixels. 1px
  hairlines and focus rects stay px deliberately — crisp at any scale, and
  the tr border exists precisely to stop selection shifting rows.
- Two skinfix traps found while writing it, one by reasoning and one by
  pixels: (a) NO blanket `.win7 button` restatement — skinfix outranks the
  skin-layer neutralizer that strips 7.css's 75x23 metrics off the ~84 icon
  buttons, so a blanket rule would re-inflate them all; (b) restating a base
  rule in skinfix also outranks 7.css's own VARIANTS of it by layer — the
  title-bar rule silently shrank the close button from 48px to 29px until the
  harness pixel diff caught it, and the active tab's padding-bottom needed
  the same treatment. Both variants are restated alongside their base rules.
- TS px constants that mirror rem CSS are the latent-bug half of this work:
  both context menus' flip/clamp math converts through a new @core/metrics
  remPx() helper reading the LIVE root font-size (the shadlike menu's
  MENU_W=224 diverged from its w-56 box the moment scale != 1; the 7like
  menu's [style.width.px] pinned the box at 200px while text grew — it now
  binds width in rem, so math and box cannot disagree). The editors'
  cachedCharWidth is now keyed on the computed font string, self-invalidating
  under any font change, and both || px fallbacks derive from the element's
  font size instead of assuming a 16px root.

Verified: tsc + build clean; modern and win7 live captures and the harness
all diff to zero against pre-change baselines in their opaque regions (the
harness to exactly zero end to end; modern's only delta is the native GTK
titlebar focus state; win7's is the translucent Aero corner over the
desktop).

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

ui/src/styles.css

@@ -181,45 +181,33 @@
   --color-icon-media: var(--icon-media);
   --color-icon-archive: var(--icon-archive);
   --color-icon-code: var(--icon-code);
-  --radius-sm: calc(var(--radius) - 4px);
-  --radius-md: calc(var(--radius) - 2px);
+  /* rem subtrahends (4px/2px at the 16px root), NOT px: --radius is rem, and a
+     mixed-unit calc would stop the derived radii from following the interface
+     scale — corners would sharpen as the UI grew. */
+  --radius-sm: calc(var(--radius) - 0.25rem);
+  --radius-md: calc(var(--radius) - 0.125rem);
   --radius-lg: var(--radius);
 }
 
-/* Interface-size setting (data-text set by SettingsService). Scales the whole
-   panel chrome: file list, path bar, action buttons, and the source picker
-   with its dropdown. The tab bar (including the options button) stays fixed.
-   Default-tier values match the old hardcoded classes exactly. */
+/* Panel-chrome metrics: file list, path bar, action buttons, source picker.
+   ALL REM, deliberately — the interface scale (settings.uiScale) works by
+   setting the root font-size, so every rem value in the app scales from that
+   one lever. A px value here would freeze while everything around it grew,
+   which is exactly what happened to the win7 boxes before stage 1c. There is
+   only one tier now: the old data-text compact/large tiers are gone, replaced
+   by the continuous scale. Values are the old default tier verbatim
+   (13px -> 0.8125rem etc. at the 16px root), so scale 1.0 is pixel-identical
+   to the pre-scale app. */
 :root {
-  --list-font: 13px;
-  --list-meta-font: 12px;
-  --path-font: 12px;
-  --row-h: 28px;
-  --path-ctl-h: 28px;
-  --action-h: 24px;
-  --action-font: 11px;
+  --list-font: 0.8125rem;
+  --list-meta-font: 0.75rem;
+  --path-font: 0.75rem;
+  --row-h: 1.75rem;
+  --path-ctl-h: 1.75rem;
+  --action-h: 1.5rem;
+  --action-font: 0.6875rem;
   --action-icon: 0.9rem;
 }
-:root[data-text='compact'] {
-  --list-font: 12px;
-  --list-meta-font: 11px;
-  --path-font: 11px;
-  --row-h: 24px;
-  --path-ctl-h: 24px;
-  --action-h: 22px;
-  --action-font: 10px;
-  --action-icon: 0.85rem;
-}
-:root[data-text='large'] {
-  --list-font: 14px;
-  --list-meta-font: 13px;
-  --path-font: 13px;
-  --row-h: 32px;
-  --path-ctl-h: 32px;
-  --action-h: 28px;
-  --action-font: 12px;
-  --action-icon: 1.05rem;
-}
 
 /* Soft entrance for the panel busy states (connecting placeholder, load overlay). */
 @keyframes panel-fade-in {