feat(inventory): multi-colour gradients, transparency, visual effects (#1154)

Spool and color_catalog rows carry extra_colors (comma-separated hex
  stops) and effect_type (14 visual variants: surface effects, sheen,
  structural). The shared FilamentSwatch component renders gradient,
  conic, effect overlay, and alpha-checkerboard consistently across the
  inventory grid, table, group banner, card, ColorSection preview, and
  catalog editor. Catalog hex_color accepts #RRGGBBAA so catalog entries
  can carry transparency too.

  The paste field accepts the exact format 3dfilamentprofiles.com puts on
  its filament details pages, so users can copy a multi-colour combo
  directly. The effect dropdown spans the full filament-variant
  vocabulary -- surface effects (sparkle/wood/marble/glow/matte), sheen
  variants (silk/galaxy/rainbow/metal/translucent), and structural
  variants (gradient/dual-color/tri-color/multicolor). None of these
  fields touch MQTT/firmware -- pure visual hint.

  Spool group-key extended to include extra_colors + effect_type so
  "Group similar" no longer collapses visually distinct spools.

  Migrations: 4 idempotent ALTER TABLE ADD COLUMN (Postgres-safe), plus
  ALTER COLUMN hex_color TYPE VARCHAR(9) on Postgres only (SQLite ignores
  VARCHAR length).

  Tests: 42 new backend (35 unit + 7 integration), 20 new frontend (14
  FilamentSwatch + 3 ColorCatalogSettings + 3 InventoryPageGrouping
  regression). 3522 backend + 1582 frontend tests pass; ruff clean.
  Localised across all 8 UI locales.

README.md

@@ -217,6 +217,7 @@ Perfect for remote print farms, traveling makers, or accessing your home printer
 - **Per-spool cost tracking** — Set cost/kg on each spool; costs are automatically calculated at print completion and aggregated to archives. Print modal shows real-time cost preview. Configurable default cost and currency in Settings.
 - **Bulk spool addition** — Add multiple identical spools at once (quantity 1–100) with a single form submission. Quick Add mode for stock spools that only need material, color, and weight.
 - Spool catalog, color catalog, PA profile matching, and low-stock alerts
+- **Multi-colour gradients, transparency, and visual effects** — Paste a comma-separated hex list (e.g. from 3dfilamentprofiles.com) to render a spool as a gradient or conic colour wheel; transparency shows through a checkerboard so the alpha you set is the alpha you see; pick a visual effect (sparkle, wood, marble, glow, matte) for the swatch overlay. Same fields are editable on the colour catalog so combos can be reused across spools.
 
 ### 🔧 Integrations
 - [Spoolman](https://github.com/Donkie/Spoolman) filament sync with per-filament usage tracking and fill level display