fix(desktop): clarify partial transcript ranges

[Sun-GLiang]

Summary

• explain when Desktop is displaying a bounded historical transcript range instead of the complete conversation
• keep "Return to latest" as the primary recovery action and expose the notice as persistent screen-reader status
• distinguish resident and unloaded prompt-rail landmarks while keeping unloaded prompts navigable
• preserve the existing 512 KiB range budget and avoid full-history hydration

Before / After

Before

• Navigating to an older prompt replaced the visible conversation with a bounded historical range without explaining that the range was partial.
• Newer persisted messages disappeared from the conversation body, and unloaded prompt-rail landmarks looked the same as resident ones.
• Users had to infer from the standalone “Return to latest” button that the missing messages were still saved.

After

• A persistent accessible notice explicitly states that the user is viewing an earlier part of the conversation and that newer messages remain saved but are not currently loaded.
• “Return to latest” remains the primary action and the notice disappears only after the latest range loads successfully.
• Prompt-rail landmarks visually and accessibly distinguish resident prompts from unloaded prompts while keeping sparse-range navigation available.
• Transcript loading remains bounded to the existing 512 KiB range budget; no full-history hydration is introduced.

Fixes #3875

Visual evidence

Production-faithful Storybook shell, captured at 1440 × 900 with the same bounded historical transcript state:

Before	After
Only the standalone “返回最新消息” action hints that newer messages are omitted; every prompt-rail landmark is rendered alike.	The persistent notice explains that this is an earlier range and that newer messages remain saved; unloaded rail landmarks are outlined.

Verification

• npm --workspace @maka/ui test — 235 tests passed
• node --test apps/desktop/dist/main/__tests__/desktop-transcript-range-store.test.js — 16 tests passed
• npm run build
• npm run typecheck
• npm run lint
• npm run format:check
• git diff --check

The regression coverage includes an eight-turn transcript larger than DESKTOP_TRANSCRIPT_RANGE_MAX_BYTES, historical and intermediate prompt navigation, return to latest, renderer range bounds, accessible notice markup, and resident/unloaded prompt-rail semantics.

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Maka analyzed the issue, implemented the UI and regression coverage, and ran the listed validation commands.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No