fix(projects): style the diff panel — 52 classes had no CSS at all

The project-detail Git panel's diff viewer emitted 52 class names that
style.css had no rule for, so it rendered as browser defaults: Working /
Staged / the base picker / ✕ Close as native OS buttons on a dark warm
panel, and a diff body with no monospace font and no +/- colouring at
all. The user's screenshot showed the toolbar; the audit (literal AND
template-literal class names) found the rest.

Adds the B1 diff-viewer block and the proj-*/tl-* rules, all on :root
tokens, following the file's existing two-button-role doctrine.

Also fixes, in passing:
- .df-commit-btn hardcoded #1a1712 where THEME.md mandates
  var(--on-accent) for text on a gold background
- .proj-pr-hidden — a "hidden" state class with no rule, so it never hid
- .proj-dot-stuck — ClaudeStatus has 'stuck' but only four of the five
  dots were styled, so a stuck session was the same colour as "no signal"
- --warn-wash: a derived token for the deletion wash, documented in
  THEME.md, so a re-skin moves the wash with --warn

Notable decisions, recorded in the CSS comments:
- .proj-tl-container gets NO height cap. Every capping mechanism tried
  (overflow-y:auto, mask-image, an ::after fade, a :has() row gate) cost
  more than the height it saved — the 5s wipe-and-re-append render loop
  cannot host a scroll position, and mask-clip:border-box ate the well's
  own border. It grew freely before this CSS existed; maxEvents=50 bounds it.
- The horizontal scroller stays per-hunk on .df-lines. Hoisting it to
  .df-file would drag the file path out of view when you scroll right.
- .df-file > .df-file-header keeps a smaller unscoped padding: the
  terminal page's .approval-preview also calls renderDiffFile().

Verified against the real DOM in a browser at 900px and 390x844 via a
throwaway harness (removed before commit), not from the CSS text alone.
typecheck exit 0; 2216 unit + 27 e2e tests pass.

docs/THEME.md

@@ -22,6 +22,7 @@
 | `--accent-2` | `#c9892f` | 深金（渐变 / hover） |
 | `--accent-soft` | `rgba(227,166,74,.15)` | 强调色半透明（选中底、chip、聚焦光晕） |
 | `--on-accent` | `#1a1305` | **金色底上的文字色**（深墨，保证对比度——金底配白字会糊） |
+| `--warn-wash` | `color-mix(in srgb, var(--warn) 24%, transparent)` | `--warn` 的 24% 半透明：diff 删除行底色。**由 `--warn` 派生**，换肤自动跟随，勿改写成 rgba 字面量（`--warn` 见 `:root` 的 Git 面板语义） |
 
 ## 状态色（语义独立，勿与强调色混用）
 
@@ -33,7 +34,7 @@
 | `--amber` | `#f5b14c` | 连接中 connecting |
 | `--red` | `#ff6b6b` | 断开 / 出错 disconnected / error |
 
-> Project 面板里的 session 状态语义（v0.6）：`working = --accent 金` · `waiting = --amber 琥珀(闪)` · `idle = 中性灰 #8b8a86` · `dirty = 暖琥珀`。
+> Project 面板里的 session 状态语义（v0.6）：`working = --accent 金` · `waiting = --amber 琥珀(闪)` · `stuck = --warn` · `idle = 中性灰 #8b8a86` · `dirty = 暖琥珀`。
 
 ## 接入约定（改主题只改这里）