feat(launcher): launch button + dep-conflict modal

Detail-panel bottom-right Launch button: switches to the selected
module via engine.switch_module if dep-check is clean, otherwise
opens a modal listing conflicts with Cancel + 'Launch anyway'. The
latter is greyed out unless engine.module.switch_module_supports_fallback
is exposed (separate engine slice). FSM adds STATE_MODAL_CONFLICT
with transitions to/from STATE_LIST via cancel/launch_anyway/esc.

Generic panels/modal.lua takes a def-table (title/body/buttons) and
returns the clicked button id from handle_click; reusable for future
modals beyond the conflict case.

panels/detail.lua

@@ -115,10 +115,22 @@ function M.render(ctx)
     if r and r.kind == "error" then ctx.list_tint = "error"
     elseif r and r.kind == "warn" then ctx.list_tint = "warn"
     end
+
+    -- L.7: Launch button at the bottom-right of the detail panel. Label
+    -- runs through ctx.t() so L.8's strings.lua picks it up once wired.
+    local label = ctx.t("launch_module")
+    local lw, lh = engine.render.measure_text(label, 18)
+    local btn_w = lw + 32
+    local btn_h = lh + 16
+    local btn_x = p.x + p.w - PADDING - btn_w
+    local btn_y = p.y + p.h - PADDING - btn_h
+    engine.render.draw_rect(btn_x, btn_y, btn_w, btn_h, 0x405080FF)
+    engine.render.draw_text(label, btn_x + 16, btn_y + 8, 18, 0xFFFFFFFF)
+    ctx._launch_btn = { x = btn_x, y = btn_y, w = btn_w, h = btn_h }
 end
 
 function M.handle_click(ctx, mx, my)
-    -- L.5: carousel arrow hit-test. L.7 will add launch-button.
+    -- L.5: carousel arrow hit-test. L.7 adds the launch-button.
     if not ctx.selected_entry or ctx.selected_entry == "news" then
         return false
     end
@@ -128,6 +140,34 @@ function M.handle_click(ctx, mx, my)
     if s and ctx.carousel_mod and ctx.carousel_mod.handle_click(s, mx, my) then
         return true
     end
+
+    -- L.7: Launch button → either switch directly (clean deps) or open
+    -- the conflict modal (warn/error deps). The "Launch anyway" path
+    -- requires engine.module.switch_module_supports_fallback (separate
+    -- engine slice); until that ships the button stays disabled.
+    if ctx._launch_btn and engine.spatial.aabb_contains_point(ctx._launch_btn, mx, my) then
+        local r = entry.status.dep_check_result
+        if r and r.ok then
+            engine.switch_module(entry.id)
+        else
+            ctx.modal_def = {
+                title = ctx.t("conflict_title"),
+                body = {
+                    ctx.t("conflict_body_line1"),
+                    string.format(ctx.t("conflict_body_line2_fmt"),
+                                  r and #r.missing or 0),
+                },
+                buttons = {
+                    { id = "cancel",        label = ctx.t("cancel") },
+                    { id = "launch_anyway", label = ctx.t("launch_anyway"),
+                      disabled = not engine.module.switch_module_supports_fallback },
+                },
+                module_id = entry.id,
+            }
+            ctx.modal_open = "conflict"
+        end
+        return true
+    end
     return false
 end