fix: compound inflation, budget planner CSV import, simplify budget manager

- Fix compound inflation: use Math.pow(1 + rate/100, yearsGap) instead of
  flat rate so multi-year gaps (e.g., 2026→2029) compound annually
- Budget Planner: add CSV import flow + Download Template button; show proper
  empty state when no base budget exists with Create/Import options
- Budget Manager: remove CSV import, Download Template, and Save buttons;
  redirect users to Budget Planner when no budget exists for selected year
- Fix getAvailableYears to return null latestBudgetYear when no budgets exist
  and include current year in year selector for fresh tenants

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/modules/board-planning/board-planning.controller.ts

@@ -1,4 +1,5 @@
-import { Controller, Get, Post, Put, Delete, Body, Param, Query, Req, UseGuards } from '@nestjs/common';
+import { Controller, Get, Post, Put, Delete, Body, Param, Query, Req, Res, UseGuards } from '@nestjs/common';
+import { Response } from 'express';
 import { ApiTags, ApiBearerAuth } from '@nestjs/swagger';
 import { JwtAuthGuard } from '../auth/guards/jwt-auth.guard';
 import { AllowViewer } from '../../common/decorators/allow-viewer.decorator';
@@ -170,6 +171,28 @@ export class BoardPlanningController {
     return this.budgetPlanning.advanceStatus(parseInt(year, 10), dto.status, req.user.sub);
   }
 
+  @Post('budget-plans/:year/import')
+  importBudgetPlanLines(
+    @Param('year') year: string,
+    @Body() lines: any[],
+    @Req() req: any,
+  ) {
+    return this.budgetPlanning.importLines(parseInt(year, 10), lines, req.user.sub);
+  }
+
+  @Get('budget-plans/:year/template')
+  async getBudgetPlanTemplate(
+    @Param('year') year: string,
+    @Res() res: Response,
+  ) {
+    const csv = await this.budgetPlanning.getTemplate(parseInt(year, 10));
+    res.set({
+      'Content-Type': 'text/csv',
+      'Content-Disposition': `attachment; filename="budget_template_${year}.csv"`,
+    });
+    res.send(csv);
+  }
+
   @Delete('budget-plans/:year')
   deleteBudgetPlan(@Param('year') year: string) {
     return this.budgetPlanning.deletePlan(parseInt(year, 10));