strategies: spending input is honoured + new "Custom" preset with guardrails

The user noticed the "Annual spending" field was a no-op for Trinity,
GK, VPW, VPW+floor — the strategies internally hardcoded the year-0
withdrawal as `initial_portfolio × initial_rate` (4% / 5.5%) and
ignored what the user typed. Two fixes:

(1) Trinity + GK now use state.initial_withdrawal (= the user's
    spending_target) as the year-0 draw. GK's guardrail anchor
    becomes the implied initial rate (initial_withdrawal /
    initial_portfolio), so the rule shape adapts to the user's
    chosen rate. Both strategies still fall back to their preset
    rate × initial_portfolio when initial_withdrawal isn't set
    (test paths). VPW and VPW+floor stay algorithmic — they're
    "withdraw-what's-sustainable" by design and don't take a
    spending input.

(2) New "custom" preset (SpendingPlanStrategy) exposing all the
    knobs:
    - initial_spend = "Annual spending" input
    - annual_real_adjust_pct = scale last year's withdrawal by N%
      each year (0 = constant real £, +0.02 = 2%/yr healthcare
      creep, -0.005 = -0.5%/yr slow-down with age)
    - guardrail_threshold_pct = if portfolio falls below X% of
      starting NW, trigger a cut (None = disabled)
    - guardrail_cut_pct = cut last year's withdrawal by Y% each
      triggered year

Adjust applies first, then guardrail cut — so a triggered year in
+2% adjust mode goes 40k → 40.8k → 36.7k.

UI: "custom" added to the strategy dropdown; when selected, three
extra fields appear (annual real adjustment %, guardrail trigger
threshold, guardrail cut size) with hints. The existing inputs
(spending, NW seed) drive year 0 across all strategies that use
them. About-the-model panel updated.

10 new tests on SpendingPlanStrategy + adjusted GK tests for the
new spending_target-aware behaviour. 209 backend tests + 7
frontend tests. mypy + ruff + tsc all pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

fire_planner/strategies/trinity.py

@@ -1,9 +1,10 @@
-"""Trinity 4% Safe Withdrawal Rate.
+"""Constant-real-£ withdrawal (the classic 4% rule shape).
 
-Bengen's seminal 1994 paper + the Trinity Study (Cooley/Hubbard/Walz,
-1998) — withdraw 4% of the starting balance in year 1, then keep the
-real withdrawal constant for the rest of retirement. In our real-GBP
-internal frame this is just "the same number every year".
+Withdraw `state.initial_withdrawal` in year 0, then keep that real-£
+amount fixed for the rest of retirement. In a 4% / £1M setup the year-0
+draw is £40k, then £40k real every year after. The strategy's
+`initial_rate` is kept only as a fallback for callers that don't feed
+`state.initial_withdrawal`.
 """
 from __future__ import annotations
 
@@ -20,5 +21,10 @@ class TrinityStrategy(WithdrawalStrategy):
 
     def propose_withdrawal(self, state: StrategyState) -> float:
         if state.year_idx == 0:
+            # Year 0 = the user's target spending. Falls back to
+            # initial_rate × initial_portfolio if no target was provided
+            # (zero or missing) for backwards compatibility.
+            if state.initial_withdrawal > 0:
+                return state.initial_withdrawal
             return state.initial_portfolio * self.initial_rate
         return state.last_withdrawal