feat: offer_type (una tantum/retainer), dedup tiers, redesigned Offerte tab

- offer_macros.offer_type ('una_tantum'|'retainer') + editor "Modalità" toggle
  (migration 0012, applied to prod)
- migration 0012 also adds UNIQUE(macro_id, tier_letter) — prevents duplicate
  tiers; ran after a one-time dedup of Web Domination's duplicate A/B/C tiers
- OffersTab redesigned: two-step assign (Offer → Tier cards with price), type
  badge "Una tantum/Ricorrente" instead of misleading "X mesi", no redundant
  "· A" when public_name == tier letter, cleaner active-offers cards
- getProjectFullDetail: availableMicros grouped by macro + defensive dedup;
  projectOffers/availableMicros carry offer_type/category/price
- proposal deck PricingSection shows offer type (fallback to duration for
  pre-existing proposals)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/proposal/assemble.ts

@@ -29,6 +29,7 @@ export type AssembledProposal = {
   offer: {
     macroId: string;
     publicName: string;
+    offerType: string; // una_tantum | retainer
     tiers: Array<{
       id: string;
       tierLetter: string;
@@ -99,6 +100,7 @@ export function assembleProposal(input: AssembleInput): AssembledProposal {
     offer: {
       macroId: input.offer.macro.id,
       publicName: offerTitle,
+      offerType: input.offer.macro.offer_type ?? "una_tantum",
       tiers,
     },
   };