…pshot service

Deduplicate two identical 14-line blocks in captureSnapshotForArchive
that fetch latest untracked paths and return confirmation errors. Both
code paths (new-untracked-after-acknowledgement and first-time-untracked)
now call the same private helper method.

Behavior-preserving: no logic change, same call sequence and return values.

Merged two separate `import type` statements from the same
`@/common/types/workspace` module into a single import line,
consistent with how other files in the codebase handle multiple type
imports from the same module.

Both getHeartbeatSettings and setHeartbeatSettings had nearly identical
object literals duplicated across two ternary branches, differing only
by the optional message field. Use conditional spread to build each
object once, removing the duplicated comment in setHeartbeatSettings.

After the HEARTBEAT_MAX_MESSAGE_LENGTH removal (5f002d8), normalizeHeartbeatMessageInput
became functionally identical to sanitizeHeartbeatMessage for its string | undefined input type.
The only distinguishing logic was the max-length assert which no longer exists.

Consolidate both callsites to use the single sanitizeHeartbeatMessage function, which
safely handles both user input and persisted config values.

The AddSectionButton component was removed from ProjectSidebar's
imports in #3099 but the component file and its test mock were left
behind. Remove both since nothing references them anymore.

Replace direct localStorage.setItem(LEFT_SIDEBAR_COLLAPSED_KEY, ...)
with the collapseLeftSidebar() helper that already exists in uiState.ts,
and remove the now-unused LEFT_SIDEBAR_COLLAPSED_KEY import.

…ppers

After the compaction handler refactor (710bda5), the production callers
were migrated to use the new *State methods directly. The old *Diffs
wrappers had zero remaining callers:

- discardPendingDiffs: no callers in production code or tests
- ackPendingDiffsConsumed: one caller in agentSession.ts, now updated
  to call ackPendingStateConsumed directly

peekPendingDiffs is retained as it's still used by tests as a
convenience accessor.

PR #3111 migrated scrollbar styling to use --color-surface-quaternary
and standard scrollbar-color, but left the old --color-scrollbar-track,
--color-scrollbar-thumb, and --color-scrollbar-thumb-hover variable
definitions in all four themes. These variables are no longer referenced
anywhere — remove the 12 dead lines.

…red utility

Extract identical number type guard functions that were duplicated across
compactionBoundary.ts, compactionHandler.ts, workspaceService.ts, and
historyService.ts into a shared src/common/utils/numbers.ts module.