feat(header): move the credits usage button into the view header

[luoxuanzao]

Summary

• The credits usage gauge — the button that opens the plan and add-on quota popover — moved from the end of the composer's icon row into the view header, between the update badge and the feedback button. Account-level usage reads better at the top, and the composer row keeps only session actions (new tab, new conversation, history).
• The popover panel now opens downward from the header instead of upward from the input footer, and keeps its right-aligned, header-width-tracked sizing so narrow sidebars clip neither edge.
• The in-composer context usage meter (ring + percentage) is unchanged.
• Documented under [Unreleased] in CHANGELOG.md (Changed).

Verification

• npm run typecheck
• npm run lint
• npm run test (167 suites, 3574 tests)
• npm run build
• npm run release:check
• npm run audit:prod
• Tested affected qodercli behavior against a real CLI when applicable — no CLI behavior is involved. The header button reuses the composer row's existing button chrome (same classes and icon sizes as the neighbouring header buttons), and the panel keeps its absolute, right-aligned placement with the containing block moving from the input footer to the header; it has not been eyeballed in a running Obsidian yet.

Safety

• No credentials, private vault content, internal URLs, or personal paths are included
• User-visible changes are documented in CHANGELOG.md