feat: copy screen ID to clipboard from Sidebar and ScreensPanel

[trmquang93]

Summary

• Adds a clickable monospace ID chip under the screen name in the Sidebar; one click copies the screen ID to the clipboard and flips the chip in place to Copied! for COPY_FEEDBACK_MS (2s).
• Adds ⧉ Copy Screen ID as the first entry in the ScreensPanel row context menu, so users can copy a screen's ID without selecting it first.
• Updates the user guide (#/docs) with a new ### Copying screen IDs for agent prompts subsection under the MCP area, including a > [!TIP] callout noting that connection/hotspot/document/data-model copy is deferred.

Closes the MVP scope of backlog item 5.2 — Copy Screen ID to Clipboard (backlog/items/5.2-copy-screen-id-to-clipboard.json). Connections, hotspots, documents, and data-models are explicitly deferred to a follow-up item.

Why

Screen IDs were not surfaced anywhere in the UI, so users working with the MCP server or AI agents had to open the raw .drawd JSON to reference a screen by ID. This breaks the visual-to-instruction pipeline that Drawd is built for. The chip-as-affordance pattern (mirroring GitHub commit-SHA chips) keeps the UI chrome-free while making IDs both visible and one-click copyable.

Implementation notes

• No new dependencies, no new hook/util — navigator.clipboard.writeText is called directly, mirroring the existing ShareModal.jsx pattern.
• Reuses COPY_FEEDBACK_MS from src/constants.js and COLORS / FONTS from src/styles/theme.js.
• Both affordances render unconditionally in read-only mode because copying is non-mutating; other gated context-menu entries below remain gated.
• Sidebar resets the idCopied flag via useEffect([screen.id]) so feedback never lingers across screen switches.

Test plan

• Sidebar: select a screen → click the ID chip → verify the chip flips to Copied! and reverts after ~2s; paste in a text field to confirm clipboard content matches screen.id.
• Sidebar: select screen A → click chip → switch to screen B during the Copied! window → verify B shows its own ID immediately (no leftover Copied!).
• Sidebar in read-only collaborator mode: chip is visible and copying still works.
• ScreensPanel: right-click a non-selected screen row → choose ⧉ Copy Screen ID → verify clipboard contains that screen's ID and the menu closes.
• ScreensPanel in read-only mode: ⧉ Copy Screen ID entry is visible and works; other gated entries remain gated.
• Keyboard: Tab to the chip → press Enter / Space → copies (real <button>, focusable).
• Docs page (#/docs): the new ### Copying screen IDs for agent prompts subsection renders with the > [!TIP] callout.
• npm run lint passes.
• npm run build passes.
• npm test — pre-existing mcp-server/ Satori failures are unchanged; all other suites pass.