Enhance tokenUsageSection in DiscoverView with time-scoped stats and richer breakdowns

[haiphucnguyen]

Context

tokenUsageSection in DiscoverView.kt currently renders a horizontal bar chart of
the top 5 models by all-time token usage, sourced directly from
TelemetryMetrics.llmTokensByInstance (a flat in-memory/file-backed map).

With Issue 1 landing (SQLite persistence + LlmUsageRepository), we can query
time-scoped data. This ticket covers enhancing the section to surface that richer data.

Current Behaviour

• All-time total shown in section header (discover.tokens.total)
• Top-5 bar chart with provider/model label, fill bar, abbreviated token count, and %
• Empty state if totalTokensUsed == 0
• "Open in new" icon navigates to System Diagnostics

Proposed Enhancements

1. Time-scope selector (This Month / This Week / All Time)

Add a compact segmented control or chip row below the section title to switch between:

• This Month (default)
• This Week
• All Time

2. Summary stat row above the bar chart
   Inside the Surface card, before the bar rows, add a compact 3-column stat strip:
   Tokens used. Calls made Avg tokens/call
   45.2K 38. 1.2K
   All values scoped to the active time period. Uses abbreviateTokens() (already exists) and LocalizationManager.formatNumber().

3. tokenBarRow — add call count column
   Extend the existing tokenBarRow composable with an additional calls: Int column to the right of the % label, so each row shows:

[model/provider] [====bar====] [45.2K] [62%] [38 calls]

4. Empty state - scope-aware copy
   Currently the empty state shows a single string (discover.tokens.empty). Update to show a scope-aware message, e.g.:
   "No token usage recorded this month"
   "No token usage recorded this week"
   "No token usage recorded yet"

Depends on #603