fix(goals): unified per-goal account color map + smaller pen toggle

User flagged two regressions: account colors didn't match between the
goal preview-card avatar stack on the index and the funding-widget
rows on the show page, and the color-picker pen toggle on the new-goal
modal still felt too big.

Color matching:

- `AccountStackComponent` (index card) used
  `Goals::AvatarComponent.color_for(account.name)` — MD5-of-name into
  the 10-color palette.
- `FundingAccountsBreakdownComponent` (show page) recently switched to
  `color_for(account.id.to_s)` — MD5-of-id.
- Same account, two surfaces, two different palette picks. Plus
  either hashing scheme can collide within a multi-account goal
  (palette has 10 colors).

Move ownership to the Goal model: `Goal#account_color_map` returns
`{ account_id => palette_hex }` for the goal's linked accounts. Sort
by `id` for a stable order across reloads, then assign
`palette[i % palette.size]`. Stable + collision-free up to 10
accounts in a single goal (a realistic upper bound — most goals
link 1-3).

Both consumers now read off the same source:

- `AccountStackComponent.new(accounts:, color_map:)` accepts a hash
  and falls back to the name-hash if no map provided (kept for
  callers that don't have a goal in scope yet).
- `FundingAccountsBreakdownComponent#color_for` reads
  `goal.account_color_map[account.id]`.
- Goal card on index passes `goal.account_color_map` to the stack.

Pen toggle:

The new-goal color-picker pen sat in a `w-5 h-5` circle with a
`border` ring + `text-secondary` icon. The border + secondary text
weight kept it loud against the avatar even at 20px. Drop the
border, drop the size another step (`w-4 h-4`), recolor the icon
`text-subdued` + `hover:text-secondary` so the affordance recedes
when not interacted with. Position shifts from `-bottom-1 -right-1`
(8px overhang) to `-bottom-0.5 -right-0.5` (2px overhang) since the
smaller circle doesn't need the larger float. Icon swaps "pen" for
"pencil" (the more conventional edit indicator across Sure).

app/components/goals/account_stack_component.html.erb

@@ -1,7 +1,7 @@
 <span class="inline-flex items-center" aria-hidden="true">
   <% shown.each_with_index do |account, i| %>
     <span class="inline-flex items-center justify-center w-5 h-5 rounded-full text-inverse text-[9px] font-semibold ring-2 ring-container"
-          style="background-color: <%= Goals::AvatarComponent.color_for(account.name) %>; <%= "margin-left: -6px;" if i > 0 %>"
+          style="background-color: <%= color_for(account) %>; <%= "margin-left: -6px;" if i > 0 %>"
           title="<%= account.name %>">
       <%= initial_for(account) %>
     </span>