fix(web): localize the context panel headline order and drop empty bar parts

The panel header concatenated a `45%` span with a `context.used` fragment, so
Chinese rendered "45% 上下文已用" against the ring's own "上下文已用 45%". The
header now renders the one localized `context.aria` sentence split around its
`{percent}` slot: each locale owns the reading's position while the reading
keeps its primary tone, and the side a locale leaves empty collapses through
`.headline:empty` instead of spending a header gap.

The bar mapped every composition row to a segment unconditionally, and
`.segment`'s 2px min-width kept each one visible, so a 0% occupancy panel
painted an ~8px filled bar over an empty context. Parts are now computed with
their widths and zero-width parts are filtered out, which also collapses the
plain and segmented branches into one map.

.agents/notes/implemented/feature/2026-08-05-composer-context-meter-breakdown.md

@@ -16,7 +16,7 @@ Three cooperating pieces, one per package boundary:
 
 `dsh-token-meter` extracts its pricing heuristic into `src/estimate.ts` — shared verbatim by the measurement service — and registers a third session projection, `contextBreakdown`, carrying `systemTokens` / `toolsTokens` / `messageTokens`. Envelope figures reprice last-wins on each `request/header` through `canonicalHeader`; the message figure folds surface appends and positional replacements over a per-node `{seq, tokens}` list, so compaction shrinks it the same way it shrinks the next request. A replace range absent from the folded surface throws: committed logs are surface-validated at append time, so an unresolvable range is log corruption, not a skippable event.
 
-`ui-conversation` moves context occupancy off the stats line (one home per fact) onto a composer-trailing `ContextMeter`: a 14px occupancy ring after the model seat fed by `contextPressure`, click-opening a panel that pairs the provider-exact percent and `~used / capacity` header with a 4px color-segmented bar and `~`-prefixed composition rows. The two vocabularies deliberately never reconcile — the ring, header, and bar length stay provider-exact while the heuristic shares only proportion the bar's colored segments and rows, each marked `~` because the fixed 4-chars-per-token heuristic systematically underprices CJK text and code.
+`ui-conversation` moves context occupancy off the stats line (one home per fact) onto a composer-trailing `ContextMeter`: a 14px occupancy ring after the model seat fed by `contextPressure`, click-opening a panel that pairs the provider-exact percent and `~used / capacity` header with a 4px color-segmented bar and `~`-prefixed composition rows. The two vocabularies deliberately never reconcile — the ring, header, and bar length stay provider-exact while the heuristic shares only proportion the bar's colored segments and rows, each marked `~` because the fixed 4-chars-per-token heuristic systematically underprices CJK text and code. The header is one localized sentence (`context.aria`, shared with the ring's accessible name) split around its `{percent}` slot, so each locale owns the reading's position — English leads with it, Chinese trails it — while the reading keeps its own tone; a bar part whose width computes to zero is dropped rather than rendered, because `.segment`'s min-width would otherwise paint a filled sliver at 0% occupancy.
 
 ## Alternatives considered