fix(chat): context meter reflects compression via session.context_breakdown RPC (#759)

* fix(chat): context meter reflects compression via session.context_breakdown RPC

The chat context meter read GET /api/sessions/{id} input_tokens — a
cumulative lifetime counter that compression never resets, so the meter
only ever grew and sat red after the first compaction.

Mirror the desktop: read live occupancy from the session.context_breakdown
WS RPC (context_used = compressor last_prompt_tokens, falling back to an
estimate of the live system prompt + tools + compacted history). The
compressor zeroes last_prompt_tokens when it runs, so the meter now drops
right after auto-compaction or /compress, exactly like desktop/web.

REST getSessionDetail is kept solely to feed the detail sheet's cumulative
token accounting. RPC failures keep the last known values; the meter never
blanks. Fixes #756.

* feat(chat): compressed xN badge on the context widget

The context chip now shows how many times the current session has been
context-compressed, sourced from the session.usage WS RPC (compressions =
context_compressor.compression_count) — the same usage snapshot the desktop
status bar reads. The badge explains a meter drop after auto-compaction or
/compress: the widget no longer just shows the number, it shows the story.

- UsageSnapshotResponse model + parseUsageSnapshot (Number-safe like the
  breakdown parser — WS parser decodes JSON numbers as Double)
- compressionCount added to ChatUiState, reset on session switch/create/branch
- session.usage fires alongside session.context_breakdown in the 5s poll;
  failures keep the last known badge value (best-effort, never blank)
- 3 parser tests; full local suite 615 tests green

---------

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/java/com/m57/hermescontrol/data/ws/WsMethods.kt

@@ -15,6 +15,12 @@ object WsMethods {
     const val SESSION_TITLE = "session.title"
     const val SESSION_BRANCH = "session.branch"
 
+    /** Live context-window occupancy for the chat meter (desktop-mirror). */
+    const val SESSION_CONTEXT_BREAKDOWN = "session.context_breakdown"
+
+    /** Lifetime usage snapshot (calls, token totals, compression count). */
+    const val SESSION_USAGE = "session.usage"
+
     // ── Interaction ───────────────────────────────────────────────────────
     const val PROMPT_SUBMIT = "prompt.submit"
     const val CLARIFY_RESPOND = "clarify.respond"