feat(canvas): export canvas dashboards as PDF

[nishantmonu51]

Adds a PDF tab to the share popover for canvas dashboards, allowing users to export the rendered canvas as a downloadable PDF.

• New web-common/src/features/exports/pdf/ module: captures the canvas, expands tables, settles async rendering, lays out pages, and assembles the PDF via jspdf.
• ShareDashboardPopover gains a "PDF" tab, shown only for canvas dashboards (when canvasName and instanceId are provided).
• CanvasDashboardWrapper renders an off-screen CanvasPdfExportHeader used solely as the PDF capture target.
• Adds jspdf dependency.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

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Developed in collaboration with Claude Code