feat(frontend): unlocks/inventory UI, sigil designer, listening-tool EVP threshold

Workstream F of the character-depth/ghost-log spec.

- InventoryPanel.tsx: essence balance, owned unlocks/items (terminal
  listing + Codex-style rarity-glow borders on items), and a buy flow
  for a small fixed unlock catalog (currently just "listening_tool",
  the only key the contract names) with afford/can't-afford button
  states. Prices are fetched from a best-guess /api/inventory/catalog
  endpoint and fall back to a flagged "(est.)" price sourced from the
  spec's own worked example when that endpoint isn't available yet —
  the contract doesn't define a price-list REST shape.

- SigilDesigner.tsx + lib/sigil.ts: constrained geometric builder —
  points snap to 24 fixed clock-face slots around a circle, capped at
  12 to match the backend's payload limit, connected in placement
  order. Five hand-drawn stroke-only rune glyphs (eye/crescent/key/
  spiral/thorn) overlay the center. Point-cap enforcement, rune
  selection and payload-shape building are pure functions in
  lib/sigil.ts, unit-tested directly. Saves via POST
  /api/inventory/sigils (path inferred; payload shape matches the
  contract exactly: {"points": [[x,y],...], "rune": str}). Art
  direction: the builder itself reads like a plotting/debug tool
  (crosshair cursor, monospace coordinate HUD) while the rendered
  lines + rune glow violet, consistent with GhostGlyph's conventions.

- lib/evp.ts: new evpThresholdDb(hasListeningTool) pure function and
  EvpListener.start() now accepts { hasListeningTool } to lower the
  EVP anomaly threshold (8dB -> 4dB) when the unlock is owned — wired
  from useAuth().user.unlocks in SeancePage's EvpPanel, a real
  gameplay effect on which faint signals register as anomalies.

- api.ts User type gains unlocks/essence per the contract's /auth/me
  extension; new InventoryPage.tsx mounts both components behind auth
  at /inventory, linked from the séance nav.

166/166 tests pass (137 pre-existing + 29 new), i18n coverage check
clean, tsc -b clean.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/i18n/en.json

@@ -13,6 +13,7 @@
     "home": "the threshold",
     "codex": "codex",
     "shop": "shop",
+    "inventory": "inventory",
     "seance": "séance",
     "enter": "enter",
     "logout": "depart"
@@ -333,6 +334,43 @@
     },
     "honesty": "the Ultimate Quantum Box is in active development — this is a reservation manifest, no payment is taken. the website works fully without it; hardware only knocks louder."
   },
+  "inventory": {
+    "back": "← back to the séance",
+    "pageTitle": "THE SHELF",
+    "pageSubtitle": "what the veil has left you, and what it will still trade for",
+    "title": "inventory",
+    "essenceLabel": "essence",
+    "unlocksTitle": "unlocks",
+    "noUnlocks": "nothing unlocked yet",
+    "ownedTag": "[OWNED]",
+    "lockedTag": "[LOCKED]",
+    "priceValue": "{{price}} essence",
+    "priceUnknown": "price unknown",
+    "priceEstimate": "(est.)",
+    "buy": "buy",
+    "buying": "buying…",
+    "buyInsufficient": "not enough essence",
+    "buyError": "the purchase failed",
+    "itemsTitle": "items",
+    "itemsLoading": "reading the shelf…",
+    "itemsEmpty": "nothing collected yet",
+    "sigil": {
+      "title": "sigil designer",
+      "canvasLabel": "circle of anchor points",
+      "slotLabel": "anchor point {{slot}}",
+      "coordLabel": "coords",
+      "hoverHint": "hover an anchor point",
+      "pointsLabel": "points",
+      "lastLabel": "last",
+      "runeLabel": "rune",
+      "nameLabel": "name",
+      "namePlaceholder": "untitled sigil",
+      "save": "inscribe sigil",
+      "saving": "inscribing…",
+      "saved": "the sigil is bound",
+      "error": "the sigil would not hold"
+    }
+  },
   "haunting": {
     "whisper": {
       "behindYou": "behind you",