feat(printing): add template tag modifiers and syntax help

[akira69]

Summary

This PR is a focused fix for label-template formatting ergonomics (issues #795 and #669) on top of Donkie/Spoolman master.

It adds formatter modifiers directly to existing template tags:

• Datetime modifiers: date, time, date_local, time_local, datetime_short, datetime_short_local
• Date-order suffixes for date-based datetime output: :ymd, :mdy, :dmy
• Numeric modifiers: round, fixed1, fixed2

Examples:

• {first_used|date:dmy}
• {first_used|datetime_short_local:mdy}
• {remaining_weight|fixed2}

Code Quality Compliance ✅

Per https://github.com/Donkie/Spoolman/wiki/Contribute#style

• ESLint (frontend) ✓
• Prettier (frontend) ✓
• Ruff (backend) N/A, frontend-only PR
• Pre-commit hooks validated ✓

Included In This PR

• Shared template-renderer support in client/src/pages/printing/printing.tsx
  • adds |modifier parsing to existing spool label templates
• Spool-label authoring UX in client/src/pages/printing/spoolQrCodePrintingDialog.tsx
  • Available Tags... button
  • one-click tag / modifier copy
  • help link to /help#template-syntax
  • modifier syntax comes from code constants, not translated strings
• Help-page documentation in client/src/pages/help/index.tsx
  • centralized template syntax examples
  • hash-anchor retry/focus fix for /help#template-syntax
  • i18n-backed help content instead of hardcoded English-only JSX

Scope Boundary

• This PR changes spool label authoring only.
• Any filament-label benefit from the shared renderer still depends on #846, because without #846 there is no filament label printing surface upstream.
• Filament dialog parity is intentionally deferred to a follow-up PR.
• This PR is separate from #875.
• This PR is adjacent to #885, but does not depend on it.

Relationship To Other PRs

• #846 is the prerequisite for any filament-label benefit from this PR.
• The filament dialog parity follow-up currently lives in akira69/Spoolman_Labels as PR #12, because GitHub cannot stack that branch directly onto this fork branch in Donkie/Spoolman.

Issues

Fixes #795
Fixes #669

Validation Completed

• npm ci
• npx eslint src/pages/help/index.tsx src/pages/printing/printing.tsx src/pages/printing/spoolQrCodePrintingDialog.tsx
• client/node_modules/.bin/prettier --check client/src/pages/help/index.tsx client/src/pages/printing/printing.tsx client/src/pages/printing/spoolQrCodePrintingDialog.tsx client/public/locales/en/common.json
• VITE_APIURL=/api/v1 npm run build
• uv run lefthook run pre-commit
• Interactive local verification on PR876 app with populated DB (6957 filaments, 3 spools)

Screenshots

Before: raw values without modifiers

After: formatted values with |fixed2 and |date

Available Tags dialog with modifier chooser

Template syntax help page

Test Checklist

Core Functionality

• Clicking Available Tags... opens the tag/modifier dialog from spool label printing
• Clicking a tag copies a ready-to-paste {tag} value and closes the dialog
• Clicking a modifier copies a ready-to-paste {tag|modifier} value and closes the dialog
• Datetime modifiers render correctly in preview, including date-order suffixes such as {last_used|date:dmy}
• Number modifiers render correctly in preview, including {remaining_weight|fixed2}

Integration

• The dialog lists spool, related filament, and related vendor fields in the correct prefixed form
• Schema-defined extra fields appear in the available-tags list with the correct extra.* paths
• The help link navigates to /help#template-syntax and lands on the Template Syntax section

Regression

• Spool print preview and QR rendering still display correctly after the modifier changes
• Save Presets succeeds from the spool print dialog
• Existing spool label templates without modifiers still render unchanged